Cataract is clouding or opacification of the crystalline lens of the eye. For most people, cataracts are a natural result of aging, though various factors have been identified which increase the risk of age-related cataract. While the precise causes and nature of cataract are not completely understood, cataract generally involves breakdown of one or more types of lens materials, such as cells and protein structures. As described in more detail below, the crystalline lens of the eye comprises elongated fiber cells, which contain proteins called crystallins. These crystallins give the “crystalline” lens its name. The cells and the proteins therein are arranged in particular ways that keep the lens transparent and let light pass through it. But as people age, some protein may clump together and start to cloud a small area of the lens, and cells may break down as well. The resulting clouding is a cataract, and over time, it may grow larger and cloud more of the lens, making it harder to see.
Cataracts are often classified as nuclear, cortical or subcapsular. A nuclear cataract forms in the nucleus, the center of the lens, and is due to natural aging changes. A cortical cataract, which forms in the lens cortex, gradually extends its spokes from the outside of the lens to the center. Many diabetics develop cortical cataracts. A subcapsular cataract begins at the back of the lens. People with diabetes, high farsightedness, retinitis pigmentosa or those taking high doses of steroids may develop a subcapsular cataract. Pure forms of cataract (with only one type present) are found more frequently in early stages of cataract, but as cataract becomes more severe, different types may be present in the same lens, producing a mixed type of cataract.
FIG. 1 shows the crystalline lens 114, which is located in the forward part of the eye. The crystalline lens has a generally circular cross-section having two convex refracting surfaces. The curvature of the posterior surface of the lens (which is nearer to the vitreous body) is greater than that of the anterior surface. The crystalline lens is suspended by a circular assembly of collagenous fibers called zonules 104. The zonules 104 are attached at their inner ends to the lens capsule (the outer surface of the crystalline lens) and at their outer ends to the ciliary body 115, a muscular ring of tissue located just within the sclera 101, which is the outer supporting structure of the eye.
As shown in FIG. 2 (and discussed in more detail below), the crystalline lens comprises a core or nucleus of primary fibers 202 surrounded by a cortex of secondary fibers 203. The crystalline lens includes a layer of epithelial cells, and the outermost part of the lens is a non-cellular membrane called the capsule 207. Posterior subcapsular cataracts form in the cortical region of the lens adjacent to the posterior lens capsule 207. Nuclear cataracts tend to form in the primary fibers 202, while cortical cataracts tend to form in the secondary fibers 203.
Age-related cataracts are believed to occur at least partially because of continued growth of the crystalline lens. During an individual's life, the crystalline lens continues to grow by epithelial cell division at the equator of the crystalline lens and formation of differentiated fiber cells from some epithelial cells. A result of this growth is that there are more and more fiber cells in the nucleus and cortex of the lens, leading to the breakdown of extracellular and intracellular proteins. The growth of the lens in the confines of the capsule causes crowding of the fiber cells, especially in the nucleus, and this crowding may cause or contribute to breakdown of fiber cells and/or protein structures.
Currently the only effective treatment for cataracts is removal of the clouded natural lens and replacement with an artificial intraocular lens. Several techniques have been used to remove the clouded lens. Currently, the most common technique is phacoemulsification, in which the clouded natural lens is broken up and removed through a small incision. A folded intraocular lens is inserted through the small incision and unfolds in the lens capsule. The eye tends to recover more quickly when a smaller incision is made. The removal of a cataractous lens and replacement with an intraocular lens has been found to be a successful strategy for treatment of cataracts, but it still has some drawbacks. For example, the artificial intraocular lens does not provide the same degree of accommodation and focusing as a healthy natural lens.
There are various ways to break up the crystalline lens for a phacoemulsification procedure. For example, a probe coupled to a source of ultrasonic power may be inserted into the eye to transmit ultrasonic vibrations to the lens. Laser energy also can be used to break up the lens. The use of laser energy for phacoemulsification is intended to break apart the entire crystalline lens, not merely to ablate a small portion of the crystalline lens.
A complication of lens replacement surgery is that cataracts may later form toward the back of the capsule. This is referred to as posterior subcapsular opacification. A variety of prophylactic and/or curative techniques have been suggested for addressing this complication of lens replacement surgery. See, for example, U.S. Pat. Nos. 4,432,751; 4,515,794; 5,370,687; 5,445,636; 5,925,617; and 6,043,237.
In addition to removal of the cataractous lens and replacement with an artificial intraocular lens, various other techniques for addressing cataract have been suggested. U.S. Pat. No. 3,971,382 (Inventor: Krasnov) discusses a method of non-surgical treatment of soft and membranous cataracts, including congenital cataracts. The method comprises the step of cutting the anterior capsule of the lens and/or pupillary membrane without perforating injury to the eye wall with a laser beam. The laser beam is passed through the cornea, the anterior chamber of the eye and pupil. The laser beam is focused onto the anterior capsule of the lens and/or pupillary membrane to form at least one hole through which the cataract substance is let out of the lens capsule/soft cataract/(sic) into the anterior chamber of the eye where the substance is gradually dissolved.
U.S. Pat. No. 4,309,998 (Inventor: Aron nee Rosa et al) discusses a process and apparatus for opthalmological surgery, wherein the apparatus includes a laser having a beam of power greater than 1012 Watts/cm2 in one or more very short pulses of duration between 20 and 400 picoseconds. The laser beam is focused by a strong conveying lens on tissue to be cut and has such low total energy that an optical puncture is produced without any notable thermal action. The apparatus employs a Q-switched YAG laser, with a helium-neon laser producing a registering or alignment beam. The laser beams are applied to a conventional slit lamp to enable them to be aligned at the target tissue, and an electronic pulse selector may be included to select the exact number of pulses to be applied to the target.
U.S. Pat. No. 4,538,608 (Inventor: L'Esperance) discusses an apparatus and technique for non-invasive surgery to remove cataracted-lens tissue from an afflicted lens. The beam output of a laser is focused to a spot of maximum power density at the anterior surface of a cataracted lens and scanned over a predetermined area or areas of the cataracted lens. The beam is selective and safe since it is diffuse as it enters the eye through the cornea and is also diffuse (being divergent) in the unlikely event that the beam passes through an opening it has created in the cataracted lens. This diffusion assures against damage to either or both of the cornea and the retina. Focal power levels are used sufficient to achieve cataract material destruction through ablative photodecomposition, thermal decomposition, photofragmentation, photoemulsification or any combination thereof. Various features are disclosed for safety and uniformity in the removal of involved tissue.
U.S. Pat. No. 5,439,462 (Intelligent Surgical Lasers) discusses an ophthalmic laser system for removing cataractous tissue from the lens capsule of an eye by phacofragmentation of the lens tissue for subsequent aspiration of the treated tissue. More specifically, a cutting laser is provided which creates a plurality of computer controlled and directed incisions in various strata through the lens tissue. Within each stratum, each incision is computer controlled and is made in the direction from a posterior to an anterior position. The strata are stacked on each other in the posterior-anterior direction, and each includes a plurality of minute incisions. The most posterior stratum of incisions is created first by referencing the cutting laser back into the lens tissue from the posterior capsule. Subsequent, more anterior strata are created by referencing the cutting layer from the tissue treated by the previous stratum of incisions. In each stratum, the vapors which result from the incisions infiltrate between the layers of the lens tissue, fragmenting and liquefying the tissue. The computer controlled device can automatically determine locations and dimensions of incisions, as well as automatically adjusting incision curvature and beam intensity as the incision point is moved from stratum to stratum. After the device liquefies the lens tissue it can then be aspirated. See also related U.S. Pat. No. 5,246,435.
U.S. Pat. Nos. 4,744,360 and 5,919,186 (Inventor: Bath) discuss a method and apparatus for removing cataracts in which a flexible line preferably 1 mm or less in diameter is inserted through an incision into the anterior chamber until its end is adjacent the cataract. Coherent radiation, preferably at a frequency between 193 and 351 nm, is coupled to the cataract by an optical fiber in the line. An irrigation sleeve provided about the fiber and an aspiration sleeve extending partially around the irrigation sleeve conducts irrigating liquid to remove ablated material from the anterior chamber and form with the optical fiber the flexible line.
U.S. Pat. No. 5,403,307 (Inventor: Zelman) discusses using a laser to soften the cataractous tissue prior to removal with a wedge-tipped probe. The cataract softening or other laser energy used during the surgical procedure being performed may be delivered through the operating microscope. In the case of cataract softening, when the laser is so delivered, no delay is required between cataract softening and cataract removal.
U.S. Pat. No. 6,811,553 (Inventor: Anthone) discusses a method to provide an efficient, safe, and easy to use supracapsular method for removal of cataracts, wherein a groove is formed in the cataract nucleus. The nucleus is cracked along the groove into two halves and rotated approximately 90 degrees. Force is applied to the proximal half to effect movement of the distal half into a stacked position relative to the proximal half. The nucleus halves along with the remainder of the cataract are then emulsified and removed. In order to minimize the chances of trauma to the capsule while sweeping the lens capsule away from cataract portions as well as making a crack in the nucleus and for otherwise assisting in manipulation of nucleus portions, an instrument has a prongless cataract-engaging portion, preferably with a convex frontal edge.
The use of laser energy for ablation of lens material has been discussed previously. U.S. Pat. No. 5,465,737 (Inventor: Schachar) and other patents issued to the same inventor describe treating presbyopia and hyperopia by a method which increases the amplitude of accommodation by increasing the effective working distance of the ciliary muscle in the presbyopic eye. Schachar states that presbyopia is also arrested by inhibiting the continued growth of the crystalline lens by application of heat, radiation or antimitotic drugs to the epithelium of the lens.
U.S. Pat. No. 6,322,556 (Gwon) discusses a method for the laser photoablation of ocular lens tissue which comprises the steps of determining a volume of the lens tissue to be photoablated and directing a pulsed, infrared laser beam at the volume with an amount of energy effective for photoablating the determined region without causing substantial damage to surrounding tissue regions. The laser beam is initially directed at a focal point below an anterior surface of the ocular lens and such focal point is moved towards the ocular lens anterior surface in order to ablate the determined volume. The laser is preferably an Nd:YLF laser operating at a frequency of about 1053 nanometers and a pulse repetition rate of about 1000 Hertz with a pulse width of about 60 picoseconds. Each pulse has an energy of about 30 microjoules. The laser operates with a focused beam diameter of about 20 microns and operates with a “zone of effect” of no greater than about 50 microns. The method is said to provide for the correction of myopia, hyperopia or presbyopia and enables the removal of incipient cataract.
U.S. Pat. No. 6,325,792 (Inventors: Swinger and Lai) describes the application of low energy, ultra-short (femtosecond) pulsed laser radiation to the patient's eye in one of a number of patterns such that the exposed ocular tissue is ablated or excised through the process of optical breakdown or photodisruption in a very controlled fashion. Using the laser inside the eye allows the surgeon to perform glaucoma operations such as trabeculoplasty and iridotomy, cataract techniques such as capsulectomy, capsulorhexis and phacoablation, and vitreoretinal surgery, such as membrane resection. The various procedures are accomplished by controlling energy flux or irradiance, geometric deposition of beam exposure and exposure time.
Cataract remains a major global affliction causing blindness in millions of people. However, there is currently no promising non-surgical therapy for cataract on the immediate horizon. Despite the numerous patents and publications describing treatments for cataracts, the current standard treatment for cataract is removal and replacement of the lens. It would be highly desirable and beneficial to be able to reduce the likelihood of cataract.